I/C chip suitable for wire bonding

ABSTRACT

A method of forming wire bonds in (I/C) chips comprising: providing an I/C chip having a conductive pad for a wire bond with at least one layer of dielectric material overlying the pad; forming an opening through the dielectric material exposing a portion of said pad. Forming at least a first conductive layer on the exposed surface of the pad and on the surface of the opening. Forming a seed layer on the first conductive layer; applying a photoresist over the seed layer; exposing and developing the photoresist revealing the surface of the seed layer surrounding the opening; removing the exposed seed layer; removing the photoresist material in the opening revealing the seed layer. Plating at least one second layer of conductive material on the seed layer in the opening, and removing the first conductive layer on the dielectric layer around the opening. The invention also includes the resulting structure.

FIELD OF THE INVENTION

The invention generally relates to manufacturing integrated circuit(I/C) chips and, more particularly, to a technique for forming wirebonded I/C chips, and the resulting structure.

BACKGROUND OF THE INVENTION

Wire bonding is well-known in the art and used widely to interconnectchips with chip carriers. Traditionally, a gold wire is bonded to analuminum pad. There are several limitations of this method. Theintermetallics formed between gold and aluminum may reduce thereliability of the bond. Additionally, the robustness of the bondingprocess is compromised by bonding dissimilar metals. In advanced CMOStechnology, copper metallization and low-k dielectrics, e.g. FSG(fluorinated silica glass), SiLK (a polyarelene ether by Dow Chemical),are used and, for even more advanced applications, porous, very weakmaterials are employed. This stage is sometimes referred to as the BackEnd of the Line (BEOL). The BEOL circuit structure is low modulus andsensitive to damage by pressure. The top aluminum surface, normallycovered by a layer of oxide, must be removed in order to form a goodcontact between the test probe and the pad metal. This probe movement iscalled “scrubbing” or “plowing”. Therefore, the chip is subjected topotential mechanical damage. It is highly desirable to establish an I/Opad that does not require scrubbing to provide low contact resistancewith test probes. Currently, because of the need to “plow” into topaluminum, the pad size has to be quite large. This limits the chipdesign for higher I/O counts or results in an increased chip size. Thepad size is often double the size needed for bonding, since the probingarea can become so damaged that it cannot be used for bonding.

SUMMARY OF THE INVENTION

A method of forming a wire bond structure in an integrated circuit (I/C)chip comprising the steps of: providing an I/C chip having a conductivepad for attaching to a wire bond with at least one layer of dielectricmaterial overlying the pad for the wire bond; then forming an openingthrough said at least one layer of dielectric material to expose aportion of said pad for said wire bond. Thereafter, forming at least afirst conductive layer on said exposed surface of said pad for said wirebond and on the surface and in the said opening in said layer ofdielectric material, and then forming a seed layer on said firstconductive layer. Then applying a photoresist material over said seedlayer, exposing and developing said photoresist layer to reveal thesurface of said seed layer surrounding said opening in said dielectricmaterial, removing the exposed seed layer, and removing the photoresistmaterial in said opening to reveal the seed layer thereunder. Thenplating at least one layer of conductive material on said seed layer insaid opening, and removing the remaining portion of said firstconductive layer of conductive material on said dielectric layer aroundsaid opening. The invention also includes the resulting structure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1–8 show schematically the steps in forming a wire bond connectionon an I/C chip according to this invention;

FIG. 9 is a photomicrograph of the plated Ni/Au wire bond pad; and

FIG. 10 is a photomicrograph of the side wall of the Ni/Au wire bondpad.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Referring now to the drawings, and for the present to FIG. 1, a portionof an I/C chip 10 at the stage of production where a wire bond pad is tobe formed is shown. Only the topmost layers and only one half of the padarea of the chip are shown. Also, the method described herein isperformed at the wafer level. This stage is sometimes referred to as theBack End Of the Line (BEOL). The chip 10 includes an SiO₂ or otherdielectric layer 12, typically about 0.5 um thick, with an Si₃N₄ layer14, typically about 0.5 um thick, thereon. A layer of aluminum 16,typically 1 to 2 um thick, is deposited on the layer 14, which Al layer16 has an extension 18 extending through opening 20 in layers of SiO₂12, and Si₃N₄ 14. Another layer of SiO₂ or other dielectric material 22,typically about 0.5 um thick, is deposited on an Si₃N₄ layer and on thealuminum layer 18, and another layer 24 of Si₃N₄, typically about 0.5 umthick, is deposited on SiO₂ layer 22. A layer of dielectric material,preferably a polyimide and more preferably a photosensitive polyimide30, typically about 6 um thick, is deposited over the layer 24 of Si₃N₄,and an opening 32 is formed through the polyimide 30, the Si₃N₄ layer 24and the SiO₂ layer 22. If the polyimide is photosensitive, preferablythe opening 32 is formed by conventional expose and develop techniques.If the polyimide 30 is not photosensitive, the opening 32 is preferablyformed by a method using laser and mask. As indicated earlier, thisrepresents the stage of formation of a portion of the I/C chip 10 whenit is ready to have the wire bond pad formed according to thisinvention. Since the processing of the chip to this point can beconventional, such processing need not be described in any detail.

FIGS. 2–8 depict the various steps in forming the wire bond padsaccording to this invention. Referring now to FIG. 2, layers of TaN/Ta36 and Cu 38 are sputter deposited by techniques, familiar to thoseversed in the art, on the layer of polyimide 30, and in the opening 32included on the exposed surface of the Al layer 16 and the sidewalls ofthe opening 32. The sputtering process is conventional, with the TaNbeing deposited first, about 100 A to 800 A, and then the Ta, also about100 A to 800 A, to form the TaN/Ta layer 36, then the Cu layer 38 (about1500 A to 5000 A). The Cu will act as a seed layer forelectro-deposition of metal, as will be described presently. It is alsobelieved that, during the sputter deposition process of the TaN/Ta, thetop surface of the polyimide 30 is carbonized to form a thin layer ofcarbonaceous material 40. Since this carbonaceous material is conductive(as opposed to the polyimide itself), when current is applied, thecarbonaceous material will add to the conductivity of the layers 36 and38, as will be explained presently.

Referring now to FIG. 3, a photoresist 44 is applied over the copperlayer 38, typically about 0.5 to 3.0 um thick, and photopatterened anddeveloped in a conventional manner to reveal all of the copper layer 38,except that which is in the opening 32. Although either positive ornegative photoresist can be used, positive is preferred since it workswell with copper, which is the seed layer.

Referring now to FIG. 4, the exposed copper seed layer is removed,preferably by electroetching. This is a conventional process andtechniques for this are shown in U.S. patents, e.g. by Datta et al inU.S. Pat. No. 5,486,282, and by Dinan et al in U.S. Pat. No. 5,536,388.

The photoresist in the opening 32 is removed by a conventional process,such as in a strong base to produce the structure shown in FIG. 5, whichhas the copper seed layer 38 remaining only in the opening 32.

Referring now to FIG. 6, layers of Ni 48, then gold 50, areelectroplated onto the revealed copper seed layer 38 in opening 32. Theelectroplating process is conventional, using the TaN/Ta layer 36, thecarbonaceous layer 40 and the copper layer 38, where it exists aselectrical conductors. Thus, the carbonizing of the surface of thepolyimide 30 has a positive, beneficial effect for electroplating.Electroplating or electrochemical deposition, is widely used in theindustry to deposit metals on wafers or substrates. A cathodic currentis applied to a wafer surface in either a nickel or gold plating tank.Based on plating time and applied currents, the amount of metal platedis controlled. The Ni is plated to a thickness of about 0.2 to 2microns, and the Au is plated to a thickness of about 0.2 to 1 micron.The plating takes place only on the Cu layer 38, and not on the TaN/Talayer 36. Also, the plating of the Au preferably takes place immediatelyafter the Ni plating to avoid any possible oxidation of the Ni. Theplating ceases when the configuration has reached that shown in FIG. 6,and only minimally overlies the surface of the polyimide 30 surroundingthe opening 32. Thus, as can be seen in FIG. 6, an augmented bond pad ofTaN/Ta/Cu/Ni/Au is provided in the opening 32 which includes the sidewalls as well as the bottom of the opening 32. Therefore, assuming thatthe size of the original Al bond pad 16 exposed in the opening 32 is 92microns×48 microns, by using the side walls of the opening, theconductive area of the original bond pad is augmented by more than 30%,e.g. about 38%.

Referring now to FIG. 7, the remaining TaN/Ta layer and remainingcarbonaceous layer 40 are removed by plasma etching in a CF₄ basedplasma using a standard etch tool to remove the TaN/Ta layer 38 and anoxygen plasma to remove the carbonaceous layer 40. The chip is now readyto receive a gold ball bond 52 and wire 54 as shown in FIG. 8. The goldbond is comprised of the gold ball 52 which is formed by heating thegold wire 54 as is commonly practiced in the art. The bonding ispreferably done by ultrasonic techniques, pressing the wire 54 and theball 52 into the bond pad. This results in a strong, electricallyefficient bond of the wire 54 to the Au of the Ni/Au layer 48 as shownin FIGS. 9 and 10. For illustration purpose, as noticed in FIG. 10, onlyone sidewall is covered with gold metal. Ideally, all sidewalls arecovered to maximize the bonding surface.

1. An I/C chip suitable for wire bonding comprising: at least oneconductive bond pad; at least one layer of dielectric material overlyingsaid conductive bond pad; a surface defining an opening in said layer ofdielectric material exposing said conductive bond pad; an intermediateconductive layer of TaN/Ta disposed in said opening overlying saidconductive bond pad and in contact therewith and in contact with theentire surface of said opening and a top surface of said layer ofdielectric material; a conductive seed layer disposed in said opening,overlying said intermediate conductive layer and in contact therewithand overlying said top surface of said layer of dielectric material,said conductive seed layer having exposed edges; two layers of Ni and Auplated on said conductive seed layer in said opening, overlying saidconductive seed layer and completely covering said conductive seed layerincluding all exposed edges, said two layers of Ni and Au define wallsin said opening in which a ball bond and wire is wholly disposed.
 2. TheI/C chip as defined in claim 1 wherein the conductive bond pad in theI/C chip is Al.
 3. The I/C chip as defined in claim 1 wherein the ballbond is Au.